Your first problem (and I think, that's the cause of the crash later) are the 150 errors. It's because these are objects, that no longer are in your wad, when you open the project. So the editor finds triggers, that refer to objects, that it can't find in the wad.

Do you have WadMerger? Ask WadMerger to make a wad from the level/demo you have. And use that wad, when you open your project. Then hopefully all of those 150 errors have gone.

The next problem is the "evil" errors. You can't ignore "evil" errors. If such errors appear, it means your level will crash. And with 20 of them this sounds really bad.

You can't just rip a level, output it again and expect it to work. First you have to clean it up. Remove walls and portals that shouldn't be there ect. If you are lucky, someone with time on their hands will offer to help you with this.

Else I would suggest you build your level from scratch and use the ripped project as a guideline to how it should look.

I'm guessing this is the prj you extracted from that .tr4 file using TR2PRJ, as mentioned in your other thread?
Boy, TR2PRJ sure does a buggy job then.

Remove walls and portals that shouldn't be there ect.

Removing ALL portals could indeed help.
The "evil object" message has been popping up on my end due to faulty portals between rooms.
And when rooms are not moved, it is easy enough to connect all the rooms again once the level works.

Otherwise, yes, remaking from scratch is probably needed then.
This faulty prj could then be used for reference if you want the exact same roosm back.

Nothing is in contradiction with nature,
only in contradiction with what we know of it.

First make sure you don't get those 150 errors. Perhaps you can use FexInspect to help you find out, what objects were in the wad, you used for the demo. FexInspect by TurboPascal will show you all objects, and where they are triggered.

Then remove all portals.

Then try to output the wad. If you don't get any "evil" errors, you should have a chance to make the level work.

Then you can add the portals again.

Then you output the wad once more. If you still don't get "evil" errors, you are ready to build on and expand your level.

To acces room 45 (the flipped room), you have to find the unflipped room. Look through your rooms and search for the room, where the F-button next to the room number is pressed (is black). In that room you press alt + f, so the area around the room becomes black. Now you are in the flipped room.

Warning: make sure you leave the flipped state, before you save. You do that by pressing alt + f again, so the area around the room turns white again.